1. Field of the Invention
The present invention relates to a piezoelectric film and a method for forming the piezoelectric film, as well as a piezoelectric device and a liquid discharge device employing the piezoelectric film.
2. Description of the Related Art
Piezoelectric devices, which include a piezoelectric material that expands or contracts when the intensity of an applied electric field is increased or decreased and an electrode for applying the electric field to the piezoelectric material, are used in applications, such as piezoelectric actuators provided in inkjet recording heads. As piezoelectric materials, PZT (lead zirconium titanate) and substitution systems of PZT, which has a part of the A-site and/or B-site thereof being substituted with a different element, etc., are known.
In view of reduction of size and thickness of piezoelectric devices, a piezoelectric material in the form of a thin film may be preferred. The piezoelectric material in the form of a thin film has a lower withstand voltage than a piezoelectric material in the form of a thick bulk body. Therefore, when the thin-film piezoelectric material is repeatedly driven with application of a voltage thereto, it is likely to suffer breakdown and be deteriorated in displacement property.
Japanese Unexamined Patent Publication No. 2003-188433 (which is hereinafter referred to as patent document 1) discloses a piezoelectric device including a substrate, and a first orientation controlling layer, a lower electrode, a second orientation controlling layer, a piezoelectric film and an upper electrode formed in this order on the substrate (see claim 1). Patent document 1 teaches that properties, such as withstand voltage, are improved by controlling the orientation of the piezoelectric film with the two orientation controlling layers (see paragraph 0118).
In Examples 1 to 3 of patent document 1, PZT films (2.6 to 3.0 μm-thick) having a piezoelectric constant d31 ranging from 120 to 128 pC/N and a withstand voltage ranging from 114 to 123 V are formed through sputtering (see paragraphs 0036 to 0078).
Japanese Unexamined Patent Publication No. 2002-370354 (which is hereinafter referred to as patent document 2) discloses a piezoelectric device which includes a lower electrode containing Pt formed on a Ti adhesion layer having a film thickness of not more than 80 angstrom (see claim 2). Patent document 2 teaches that formation of protrusions on the surface of the lower electrode is minimized with the film thickness of not more than 80 angstrom of the Ti adhesion layer, and the withstand voltage is improved (see paragraphs 0097-0098).
In Examples 1 and 5 of patent document 2, PZT films are formed through sputtering. It is shown in Example 1 that d31=150 pC/N is achieved. The relationship between the film thickness of the Ti adhesion layer and the withstand voltage is reported and evaluated in Example 5, and a withstand voltage ranging from 50 to 80 V is achieved with a 1 μm-thick PZT film (see Table 4 and paragraph 0100).
Japanese Unexamined Patent Publication No. 2008-055871 (which is hereinafter referred to as patent document 3) discloses a piezoelectric device which includes an upper electrode formed in a depression provided through a RIE process, or the like, in an upper layer portion of a piezoelectric film in order to improve the withstand voltage (see claim 1). It is shown in Examples 1 and 2 that 4 μm-thick PZT films formed through sputtering had a withstand voltage ranging from 120 to 150 V.
In these examples, however, the size of the upper electrode was 75 micrometer-wide, which is far smaller than that in specifications for practical use. It therefore seems that higher values were estimated for the withstand voltage. Although no data of the piezoelectric constant is reported, the intrinsic PZT does not provide a high piezoelectric constant. The present inventors formed intrinsic PZT films through sputtering according to the method disclosed in patent document 3, and the PZT films had the piezoelectric constant d31 around 130 pC/N.
Patent documents 1 to 3 report the PZT films formed through sputtering having a withstand voltage ranging from 50 to 150 V. However, all of the PZT films are intrinsic PZT films and have a piezoelectric constant d31 of not more than 150 pC/N. With respect to piezoelectric films, a film having a smaller piezoelectric constant d31 tends to exhibit a smaller flexural displacement measured under conditions of a fixed film thickness and a fixed applied voltage, and a higher withstand voltage. In patent documents 1 and 3, the high withstand voltage is achieved because the piezoelectric constant d31 is not high.
The PZT film of patent document 1 requires providing the two orientation controlling layers as underlayers, and the PZT film of patent document 3 requires providing the depression in the upper layer portion of the piezoelectric film. Therefore, the process is not simple.
U.S. Pat. No. 7,312,558 (hereinafter referred to as patent document 4) discloses a PZT piezoelectric film having a columnar structure, which is formed by a number of columnar particles extending in the thickness direction and having a ratio of average cross-sectional diameter to length of not less than 1/50 and not more than 1/14, with an orientation controlling layer made of a cubic or tetragonal perovskite oxide provided as an underlayer (see claim 1). Patent document 4 discloses, in table 2, etc., piezoelectric films of substitution systems of PZT doped with Nb, or the like, having a piezoelectric constant d31≧200 pc/N and a breakdown voltage of 100 V or more. However, the piezoelectric film of patent document 4 requires orientation control by the orientation controlling layer.
As another means to improve the withstand voltage, it has been proposed to provide a protective film. However, requiring a film, such as an orientation controlling layer or a protective film, which is not essential for a piezoelectric device, increases the number of process steps and complicates the process, resulting in increased production costs.